Browsing by Author "Mpetile, Ziyanda"

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    Genetic variation in and relationships among faecal worm eggs recorded in different seasons of the year at the Tygerhoek farm in South Africa
    (AOSIS Publishing, 2017-07-07) Mpetile, Ziyanda; Dzama, Kennedy; Cloete, Schalk W. P.
    ENGLISH ABSTRACT: Gastrointestinal nematodes result in severe economic and production losses to the sheep industry. An increase in resistance of the nematodes to chemicals used for control, as well as a demand of consumers for meat products free from chemicals, has fostered research on alternative control strategies. Breeding for resistance to nematodes offers an alternative to control parasitism but its effectiveness depends on genetic variation in faecal worm egg count (FWEC), an indirect measure of parasite resistance. A historic dataset of FWEC from four Merino lines subjected to natural parasite challenge was used to estimate genetic parameters for FWEC in three seasons (autumn, winter and spring) using a repeated records animal model, followed by a three-trait animal model analysis treating FWEC in different seasons as separate traits. The effects of selection line, birth year, sex, the sex x birth year interaction, season and the season x year interaction were significant when using 4994 records recorded from 1997 to 2000 (p < 0.001). The heritability of log-transformed FWEC amounted to 0.09 ± 0.02, with no contribution from the animal permanent environmental variance to the between animal variation across seasons. Three-trait heritability estimates for log-transformed FWEC amounted to 0.07 ± 0.05 in autumn, 0.13 ± 0.05 in winter and 0.19 ± 0.05 in spring. These results suggest sufficient genetic variation in FWEC to support selection for lower log-transformed FWEC. However, the best time to record data for selection is after the break of the season in winter and in spring, when sheep are stimulated by a greater intake of infective larvae from the pasture after the first rains. Genetic correlations among FWEC in the respective seasons were moderate to high, ranging from 0.55 to 0.89. Phenotypic correlations, on the other hand, were significant but lower in magnitude, ranging from 0.09 to 0.16. These results provide useful information for developing strategies for the genetic improvement of ovine resistance to gastrointestinal nematodes under Mediterranean conditions in South Africa by using FWEC as an indicator trait.
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    Genetics of ovine resistance to gastrointestinal nematodes
    (Stellenbosch : Stellenbosch University, 2019-04) Mpetile, Ziyanda; Cloete, Schalk W. P.; Dzama, Kennedy; Stellenbosch University. Faculty of Agrisciences. Dept. of Animal Sciences.
    ENGLISH ABSTRACT: This study consists of various papers on the genetic evaluation of sheep for resistance to gastrointestinal nematodes in South Africa using faecal worm egg count (FWEC) as a proxy. Outcomes from the study suggested that Merino lines divergently selected for reproduction at Elsenburg research farm did not differ for log transformed FWEC. Heritability estimates for FWEC at Tygerhoek research farm were low in autumn (0.07 ± 0.05), but moderate in winter (0.13 ± 0.05) and spring (0.19 ± 0.05), suggesting that the best time to sample FWEC for genetic improvement in parasite resistance in Mediterranean South Africa is after the break of season. Genetic and phenotypic parameters for FWEC, live-weight traits, wool traits, subjective traits and testis dimensions were estimated in the Elsenburg Merino flock. The direct heritability estimates of these traits ranged from 0.09 ± 0.04 for FWEC to 0.60 ± 0.03 for clean yield (CY). Genetic correlations of FWEC with the other traits of economic importance were not significant, ranging from -0.24 ± 0.18 with crimps per 25 mm to 0.30 ± 0.23 for weaning weight, suggesting that selection for parasite resistance is unlikely to hamper genetic progress in other trait complexes. Genetic variation of FWEC was estimated in mature reproducing Elsenburg Merino ewes and correlated with number of lambs born and weaned. The heritability of FWEC was low at 0.03 ± 0.03, implying genetic change in FWEC of mature ewes would be difficult. Heritability of number of lams born and weaned was respectively 0.19 ± 0.03 and 0.11 ± 0.03. FWEC and reproduction traits were favourably correlated. An across-flock genetic evaluation of FWEC in South African Merinos at both Elsenburg and Tygerhoek yielded a heritability estimate of 0.11 ± 0.02 for FWEC. The sire x site/season variance ratio was low at <2%, suggesting minimal reranking of sires across flocks. Further studies suggested an overall breed difference for FWEC between Dormer and SA Mutton Merino yearlings and breeding ewes although the results were not consistent across years. Genetic parameters for FWEC and the Famacha© traits were investigated in Merinos at Tygerhoek and Dormers and SA Merinos at Elsenburg. All traits were heritable, suggesting that genetic improvement would be feasible should it be desired. Genetic correlations suggested that selection for lower eye scores would result in a change in FWEC at Elsenburg. Genetic correlations also suggested that animals with anaemic eye scores of 4 would be more likely to have high FWECs at Elsenburg, where haematophageous helminths were more likely because of irrigated pastures. The study provided useful information on genetic parameters for FWEC as indicator of host resistance to gastrointestinal nematodes and provided the background needed for planning further research on this important input trait.